You can create a field control in your Sitefinity CMS web application or class library. 

1. In the Solution Explorer, select the folder where you want to put your field control
2. Open its context menu and click Add » New Item… 
3. In the left pane, select Sitefinity CMS and in the central pane, select Sitefinity CMS Field Control.
   Sitefinity CMS Thunder generates all the required files for your field control and, if it is located in a class library, registers it in the virtual paths configuration in Sitefinity.

You can use the field controls in both your module definitions or as fields in the module builder.